1. Field of the Invention
The present invention relates to the field of telephone line interface equipment, and more specifically to direct connect interface equipment.
2. Prior Art
Standard telephones operate on a two-conductor line (i.e., signal and return lines, though neither is necessarily at ground potential) through which all the signals required for dialing, ringing, answering and communication, and hang up are transmitted. Generally speaking, the ring signal is comprised of a DC voltage on the order of 50 volts, with AC ring signals impressed thereon of approximately 20 Hz to provide an overall signal on the order of 70 volts RMS. When the receiver goes "off hook", whether by manual pick-up of the receiver or by automatic answering equipment, an impedance of a few hundred ohms is imposed across the line by the receiver. The step-up in line current is detected at the central office after which time the ring signal is terminated, with the line connections being maintained by the central office until one of the parties hangs up as indicated by the removal of the load from the line of the calling or called party. For outgoing calls, the central office senses a receiver going off hook by an imposition of a load on the respective line and, noting that the off hook condition was not preceded by a prior connection as it would be for an incoming call, imposes a dial tone on the line to signal the caller to proceed with the dialing sequence.
An off-hook condition is generally recognized at the central office by the detection of a line current on the order of 20 ma or more, thereby requiring the imposition of a load on the line at the receiver on the order of a few hundred ohms. If simple resistive loads were used, the loads would also load down the information signals superimposed on the DC signal so as to grossly attenuate the AC signal component containing the information being communicated. Accordingly, in automatic phone answering equipment, it has been common practice to utilize an isolation transformer having a primary resistance on the order of 200 ohms and being of sufficient size so as to not substantially saturate due to the DC current therethrough (and of course the AC component riding thereon representing the information being transmitted or received). The primary of the transformer provides the desired DC load on the line though the inductance of the primary provides a relatively high impedance for the frequencies of the information on the line so as to not substantially attenuate these signals. Thus a transformer of a suitable size and design will provide the desired DC load to hold the line connection but will not substantially attenuate the information signals, and at the same time will provide the required DC isolation between the line and the answering equipment.
The DC current in the primary of the transformer coupled across the line will tend to saturate the iron in the transformer if the transformer is not of sufficient size and design. Thus, as an alternative in the prior art, a simple inductance having the appropriate DC resistance is coupled across the line, the inductance of course being designed to also not substantially saturate due to the DC current therethrough. This inductance, commonly referred to as a holding coil, will hold the line connection, with the signals being picked up from the line by an isolation transformer having the primary thereof capacitively coupled to the line. The capacitive coupling of the isolation transformer eliminates the DC component from the transformer primary, allowing a much smaller and less expensive transformer to be used. The advantage of the smaller isolation transformer is substantially diminished however, because of the required size of the holding coil to achieve the desired purpose.